Investigatating Pitting And Crevice Corrosion Behavior Of Nickel Conservation Duplex Stainless Steel

Duplex stainless steels (DSSs) is a kind of iron-based multiphase alloys which consist approximately equal amount of ferrite and austenite phases. The unique two-phase microstructure character bring DSSs with superior properties such as higher strength as well as toughness than ferrite stainless steels and austenite stainless steels. The corrosion behavior of DSSs are influenced by alloying elements (Cr, Mo, and N) contents in ferrite phase and austenite phases. Thus, the effect of microstructure evolution on the corrosion behavior of DSSs is significant. Recently lean DSSs have been developed by introducing more nitrogen as alloying elements to further improve the cost efficiency. DSS2101is a kind of lean duplex stainless steel with high corrosion resistance as well good mechanical properties that have attracted much attention from end users and producers of stainless steels. Consided that the failure of stainless steels are mainly caused by the localized corrosion, e.g., pitting and crevice corrosion, it is important to evaluate the pitting and crevice corrosion resistance of duplex stainless steel (DSS) during its application processes.DSSs are spontaneous passive alloys that passive film can be formed on the surfaces to protect themselves against corrosion attack. Nevertheless, some unwanted secondary phases (carbides, nitrides, σ,x and R) of DSSs are prone to form during exposure to temperatures between300and1300℃and their corrosion resistance are determined by the weakest phase. Thus, it is extremely important to clarify the underling relationship between alloy element distribution, secondary phase precipitation and the associated corrosion resistance. In addition, there is still controversy over the mechanisms of crevice corrosion on DSSs. Based on the complexity in DSSs, this work have been done from three stages:1. Electrochemical techniques have been used to characterizes the semiconductor properties of passive films formed on the DSSs. The development and breakdown of passive film should be accessed;2. The effect of processing techniques, such as laser beam welding and post welding heat treatments on the corrosion behavior of DSSs should be addressed;3. The mechanism of crevice corrosion on DSSs should be clarified, direct evidence should be provided to improve the understanding of crevice corrosion mechanism.Based on above review, this work firstly studied the semiconducting behavior of passive films on DSSs in neutral solutions. The development and breakdown of the passive films are characterized and analyzed base on point defect model. Secondly, the effects of chemical composition and heat treatments on microstructure evolution and the pitting corrosion resistance of a newly developed duplex stainless steel UNS S382441was studied. Which is useful for as a guidance for the development of DSSs. Third, the effect of laser beam welding on the pitting corrosion resistance of DSS2205was revealed. A systematic study of a short-time post welding heat treatment on the pitting corrosion resistance of DSS2205was performed. Conclusions and suggestions have been made to provide a guidance for an effective method to regain corrosion resistance of the welding joints. Finely, crevice corrosion of DSS2101was directly monitored and characterized by a facile method. Both the delayed and immediate crevice corrosion mechanisms were studied. The main contents and highlight of this work are as follows:1. The passive films of UNS S32101duplex stainless steel (DSS2101) in0.2M borate buffer solution (pH=8.3) at the steady state has been investigated by potentiodynamic polarization, electrochemical impedance spectroscopy and Mott-Schottky analysis. The study shows that the passive film form on DSS2101is an n-type semiconductor with a donor density which is approximately1020cm-3and donor densities evaluated from Mott-Schottky plots increased with formation potential. The steady passive current density is found to be voltage independent while the thickness of the barrier layer depends linearly on the formation potential. General corrosion rate of DSS2101at given conditions was calculated to be0.15μm/a.2. The pitting corrosion behavior of UNS S82441duplex stainless steel annealed at six different temperatures ranging from1000℃to1130℃for1h has been investigated by means of potentiostatic critical pitting temperature. The microstructure evolution and pit morphologies of the specimens were studied through optical/scanning electron microscope. The results demonstrated that the volume fraction of austenite phase decreased with increasing annealing temperature. Lower critical pitting temperature values were obtained after annealing at higher temperatures. Pitting was initiated preferentially inside the ferrite domains, indicating that the ferrite phase had inferior pitting corrosion resistance as compared to the austenite phase. The pitting resistance equivalent number of the ferrite phase fell with the annealing temperature, while the values for the austenite phase rose. No equal pitting resistance equivalent number of the ferrite and austenite phases was found for this steel in the range of annealing temperatures.3. The effect of laser-beam welding and subsequent short-time post-weld heat treatments at different temperatures and holding time on microstructure evolution and pitting corrosion behavior of UNS S31803duplex stainless steel was investigated. The results showed the as-welded joint displayed impaired pitting corrosion resistance and that pitting preferentially occurred at ferrite grain in the fusion zone. Short time heat treatment enhanced the pitting corrosion resistance of welded joint. Autogenous LBW resulted in non-equilibrium microstructure in FZ of DSS2205with α-phase volume fraction more than90%. Cr2N precipitated in the α-phase of FZ which has been confirmed by TEM and AFM. The as-welded DSS2205samples exhibited severely impaired pitting corrosion resistance with CPT value decreasing from56℃(base metal) to42℃.Stable pits occurred preferentially at α-phase in the FZ due to the unbalanced microstructure and the Cr depleted zone induced by Cr2N. Short-time PWTH improved the corrosion resistance of the welded joint and more austenite was formed in FZ compared with the as-welded state. For the3min PWHT, as the annealing temperature increased in the range of1020℃-1100℃, the ferrite volume fraction in FZ reached its lowest value at1080℃. As the annealing temperature increased from1020℃to1080℃, the CPT value increased to55℃, but further increasing the annealing temperature decreased the CPT value. To restore the pitting corrosion resistance of the welded joint, the optimum short-time PWHT parameters were determined:3min heat treatment at1080℃.4. The crevice corrosion behavior of UNS S32101was investigated by in-situ and ex-situ characterization. Experimental results revealed the initial corrosive attack occurred as a pit at the closest contact on the crevice wall. The measured current density increased gradually in the passive range and increased sharply as the pit was stabilized. Delayed and immediate crevice corrosions can be initiated by potentiostatic polarization at Eapp=0.30V and Eapp=0.50V, respectively in neutral0.1mol/L NaCl solutions at room temperature. The transition from induction stage to propagation stage of the delayed crevice corrosion was explained by IR mechanism with variation of the crevice electrolyte compositions. The relocation of active dissolution region on the crevice wall that contributed to the diversity of crevice corrosion morphology, was explained by the effects of corrosion products based on the IR mechanism. The current fluxes caused by passive/active transition of passive films on the crevice wall was considered as the main reason for the immediate crevice corrosion on UNS S32101based on IR mechanism.